1. Field of the Invention
This invention relates to a treatment for stabilizing a ferromagnetic metal powder, and more particularly it relates to a treatment for stabilizing a ferromagnetic metal powder composed mainly of iron.
2. Descripton of the Related Art
In recent years, as a magnetic material for magnetic recording media such as audiotape, video tape, video disk, etc., ferromagnetic metal powder, particularly a ferromagnetic metal powder composed mainly of iron has been noted and used. Such a ferromagnetic metal powder is obtained by heat-reducing iron oxyhydroxide or iron oxide obtained by heat-dehydration of iron oxyhydroxide in a reducing gas, and as compared with conventional iron oxide magnetic materials, it has a higher coercive force (Hc) and a larger saturation magnetization (.sigma.s) so that it is possible to achieve a higher density recording.
However, such a ferromagnetic metal powder used for magnetic recording media has a large specific surface area and is chemically very active; hence it has a drawback that when it is taken out, as it is, into the atmosphere, heat generation or ignition due to a rapid oxidation reaction occurs.
In order to overcome such a drawback, there has been proposed a process wherein a ferromagnetic metal powder is subjected to a stabilization treatment by contacting the metal powder with an oxygen-containing gas in a liquid phase or in a gas phase and gradually forming an oxidized coating on the surface of the particles of the metal powder (hereinafter such a gradual formation of an oxidized coating on the surface of the particles of the metal powder being referred to as "gradual oxidation"). For example, the so-called liquid phase gradual oxidation process wherein the powder is immersed in an organic solvent and subjected to oxidation treatment in a solution while an oxygen gas-containing inert gas is passed therethrough (Japanese patent application laid-open No. sho52-85054/1977) and the so-called gas phase gradual oxidation process wherein the powder is contacted with an oxygen-containing inert gas having the oxygen partial pressure therein adjusted to subject it to stabilization treatment (Japanese patent application laid-open No. sho48-79153/1973) have been proposed. However, since the former process is directed to a reaction in a liquid phase, it has the following drawback: in order to carry out a uniform gradual oxidation, the mass of the ferromagnetic metal powder should be ground into primary particles or secondary particles in an organic solvent, but the surface energy of the ferromagnetic metal powder is so high that the grinding contrarily forms an agglomerate, which is gradually oxidized as it is so that this has a bad influence upon the dispersibility of the ferromagnetic powder or the diffusion of oxygen dissolved in the organic solvent onto the surface layer of the ferromagnetic metal powder particles is insufficient; hence the oxidation reaction occurs nonuniformly to make the magnetic characteristics inferior. On the other hand, since the latter process is directed to a reaction in a gas phase, it has the following drawback: the activity of the ferromagnetic metal powder particles is so high that control of the oxidation reaction accompanied with heat generation is very difficult; hence this is liable to result in an nonuniform gradual oxidation, which is accompanied with reduction in the coercive force and also notable reduction in the saturation magnetization with the lapse of time.
In place of the above processes, a process has been proposed wherein contact of the ferromagnetic metal powder with an oxygen-containing gas is gradually changed from that in a liquid phase to that in a gas phase (Japanese patent application laid-open No. sho59-170201/1984). According to such a process, however, the reaction temperature is so high and the oxygen concentration in the oxygen-containing gas is also so high that the time of the oxidation reaction is very short; hence uneveness of the resulting oxidized coating due to nonuniform oxidation is liable and occur to result in deterioration in the coercive force so that when the resulting product is made up into a tape, the relative squareness (Rs) of the tape is notably reduced and also control of the saturation magnetization is very difficult.